Sweetening is the main process to which natural gas streams are subject to for the purpose of removing the acid gases it contains. The removal of said gases is essential for many reasons; one of those reasons is that they decrease the heat capacity of CH4. CO2 and H2S are highly corrosive acid gases; additionally, CO2 is considered one of the main greenhouse effect gases that contribute to global warming. On its part H2S is extremely toxic and fatal even in low concentrations. The conventionally known process used for the removal of acid gases implies chemical adsorption with amines. However, this process requires subsequent desorption to release the gases that had been removed, which are then burned or taken to the Claus process to recover the elemental sulphur and water. The technology used to selectively separate the acid gases using membranes is currently an attractive option to carry out the natural gas sweetening.
For the case of H2S separation from natural gas streams using membranes, literature is scarce. However, certain references can be found for this separation using copolymers of Poly(ether-urethane) and Poly(ether-urethane-urea) written by Chattarje et al [Chatterjee G., Houde A. A., Stern S. A., (1997), Poly(ether urethane) and Poly(ether urethane urea) Membranes with High H2S/CH4 selectivity, Journal of Membrane Science, 135, 99-106] who prepared the membranes with different polyethers and they found that it is possible to achieve separation factors close to 100 for H2S/CH4 in mixtures containing different CH4:CO2:H2S concentrations. This factor indicates that around 100 H2S molecules would flow through the membrane for every CH4 molecule, thus effectively concentrating the CH4 in the injection section of the gas, and receiving a mixture with very high H2S concentration at the membrane's bottom part. In another report about the separation of the same gases using a polyphosphazene membrane, permeability results report H2S/CH4 separation factors of about 75 [C. J. Orme, F. F. Stewa, (2005) Mixed gas hydrogen sulfide permeability and separation using supported polyphosphazene membranes, Journal of Membrane Science, 253, 1-2, 243-249].
Other studies have focused on aided transport processes in which the membranes are formed, for example, from a sulfonated ionic polymer containing an alcohol amine dissolved in the material, which acts as a carrier within the membrane to take the CO2 & H2S acid gases and transport them through the membrane [J. D. Way, R. D. Noble, (1989) Competitive facilitated transport of acid gases in perfluorosulfonic acid membranes, Journal of Membrane Science, 46, 2-3, 309-324]. Recently, Huang J. et al [Huang J., Zoe J., Winston Hoe W. S., (2008), Carbon Dioxide Capture Using CO2-Selective Facilitated Transport Membrane, Ind. Eng. Chem. Res., 47, 1261-1267] describe the aided transport of CO2 through the use of amines in a crosslinked matrix of vinyl alcohol, PVA, with amino groups joined to the polymer chain and free amines within the matrix. These membranes show good selectivity for the separation of H2S from the natural gas streams; however, given the nature of the aided transport, the flow through them takes a long time, which makes their use unfeasible for the separation in large volume streams of natural gas. Because of this, membranes with high flow of gas and capable of separating H2S from CH4 in ratios of at least 30 to 1 shall be included.
There is a larger number of reported works for the case of separation of CO2 from natural gas streams. Recently, Lin H. et al. [Lin H., Freeman B. D. (2005) Materials selection guidelines for membranes that remove CO2 from gas mixtures. Journal of Molecular Structure 739:57-74], published a revision about the selection of polymer materials with the capacity to form membranes for the separation of CO2 from gas mixtures. They suggest that in order to improve the permeability and selectivity properties of polymer membranes, the interaction between CO2 and the polymer needs to be increased through the incorporation of several polar groups; suggesting that the ethylene oxide (EO) units can be useful groups to achieve this objective. However, the poly(ethylene oxide) (PEO) shows a strong tendency to crystallize, consequently, it shows low permeability for gases [Lin H., Freeman B. D. (2004) Gas solubility, diffusivity and permeability in poly(ethylene oxide). Journal of Membrane Science 239:105-117].
Some block copolymers that contain polyether segments (EO units) have been studied as alternative materials to improve CO2 transport properties [Car A., Stropnik C., Yave W., Peinemann K-V. (2008) PEG modified poly(amide-b-ethylene oxide) membranes for CO2 separation. Journal of Membrane Science 307:88-95.; Chen J. C., Feng X., Penlidis A. (2004) Gas permeation through poly(ether-b-amide) (PEBAX 2533) block copolymer membranes. Separation Science and Technology 39:149-164; Liu L., Chakma A., Feng X. (2004) Preparation of hollow fiber poly(ether block amide)/polysulfone composite membranes for separation of carbon dioxide from nitrogen. Chemical Engineering Journal 105:43-51.; Barbi B., Furani S. S., Gehrke R., Scharnagl N., Stribeck N. (2003) SAXS and the gas transport in polyether-block-polyamide copolymer membranes. Macromolecules 36: 749-758.; Kim J. H., Ha S. Y., Lee Y. M. (2001) Gas permeation of poly(amide-6-b-ethylen oxide) copolymer. Journal of Membrane Science 190:179-193.; Bondar V. I., Freeman B. D. Pinnau I. (2000) Gas transport properties of poly(ether-b-amide) segmented block copolymers. Journal of Polymer Science. Part B: Polymer Physics 38: 2051-2062]. In the copolymers known as PEBAX®, that contain an aliphatic polyamide as rigid segment (Ex: Nylon-6, Nylon-12, PA) and a polyether as a soft segment (Ex: poly(ethylene oxide, PEO, or poly (tetramethylene oxide, PTMO)). It has been reported that PA segments provide the mechanical properties, while gas transport occurs through the PEO segment [Bondar V. I., Freeman B. D. Pinnau I. (2000) Gas transport properties of poly(ether-b-amide) segmented block copolymers. Journal of Polymer Science. Part B: Polymer Physics 38: 2051-2062].
In light of the results obtained by incorporating EO units in aliphatic polyamides, other researchers have reported the transport properties of CO2 in segmented polyether (PEO) based membranes that are copolymerized with other polymer systems [Muñoz D. M., Maya E. M., de Abajo J., de la Campa J. G., Lozano A. E. (2008) Thermal treatment of poly(ethylene oxide)-segmented copolyimide based membranes: An effective way to improve the gas separation properties. Journal of Membrane Science 323: 53-59; Yoshino M., Ito K., Kita H., Okamoto K-I. (2000) Effects of hard-segment polymers on CO2/N2 gas-separation properties of poly(ethylene oxide)-segmented copolymers. Journal of Polymer Science: Part B: Polymer Physics 38:1707-1715].
In some processes, such as the one described in the patent EP2234697, a configuration that involves, at least, a two-step separation sequence in membrane units to produce a relatively pure methane gas stream with less content of pollutants than the entering gas is required. In these cases, the multi-step separation is justified by the fact that there is a lower loss of hydrocarbons in the permeated stream, as described in the publication of the international patent application WO2012012129. In the publication of the patent application US 20060042463, even though the permeated gas is recycled through a second membrane system, the result is still an acid stream with relatively high methane content, about 30% to 50% molar, which is used as fuel for an electric power generator.
On the other hand, in the publication of the patent application US20070272079, the membranes used in the separation steps of natural gas are selective for the passage of CO2 showing high permeability for CH4, at least in a 10/1 GPU ratio, which results in the recovery of up to 90% of methane gas. However, a well known fact is that natural gas streams are contaminated with both CO2 and H2S; consequently, it is essential to consider the removal of H2S in order to obtain a CH4 stream at the conditions required for commercial use.
On the other hand, the use of plasma for the production of hydrogen from the decomposition of H2S has already been implemented at a semi-industrial level by Balebanov et al in Orenburg, Russia [Balebanov A. V., Givitov V. K., Krasheninnikok E. G., Nester S. A., Potapkin B. V., Rusanov V. D., Samarin A. E., Fridman A., Shulakova E. V., (1989), High Energy Chemistry (Khimia Vysokikh Energij). Sov. Phys., 5, 440], using systems consisting of 4 microwave discharge ports whose power is 250 kW resulting in 1 MW of total net power. After the discharge zone, the reactor has a condenser attached to it where the elemental sulphur, H2 gas, and residual H2S are separated. Its performance depends on its adaptation to the purification process of natural gas with amines and it can even be linked to the Claus process.
Likewise, the pulsed corona discharge reported by Zhaoa G. et al. [Zhaoa G. B., Sanil J., Ji-Jun Z., Hamannb J. C., Muknahallipatnab S. S., Stanislaw L., Ackermana J. F., Argyle M. D., (2007), Production of hydrogen and sulphur from hydrogen sulfide in a non-thermal-plasma pulsed corona discharge reactor, Chemical Engineering Science, 62, 2216-2227] and Sanil J. et al. [Sanil J., Hamann J., Suresh S., Legowski S., Ackerman J. F., Argyle M. D., (2009), Energy Efficiency of hydrogen sulfide decomposition in a pulsed corona discharge reactor, Chemical Engineering Science, 64, 4826-4834], has been used for the treatment of H2S. This treatment is basically carried out in a quartz reactor to which an Argon-Nitrogen mixture is added to achieve a moderately efficient decomposition of H2S. This method has limitations with respect to the low concentration of H2S (<2% mol), high energy consumption (>100 eV/H2S) and the need to add mixtures with other gases (Helium, Nitrogen, Hydrogen, and Argon) in order to increase its energy efficiency.
Another method used to treat H2S using plasma technology, is the discharge by radio frequency (R.F.), which consists of generating a high frequency signal (MHz) at very low pressures (from 10 to 50 kPa) in order to achieve non-equilibrium conditions. Some developments reported by Potapkin et al. [Potapkin B. V., Strelkova M. I., Fridman A. A., Harkness J. B. L., Doctor R. D., (1995), Mechanism and kinetics of H2S—CO2 mixture dissociation in plasma of a microwave discharge, in: J. V. Heberlein, d. W. Ernie, J. T. Roberts, (Eds.), Proceeding of the 12th International Symposium on Plasma Chemistry, 1737-1742] about the decomposition of an H2S—CO2 mixture in a magnetically coupled R.F. plasma, show a theoretical dissociation rate of 91%, applying 2.3 kWh/m3 of specific energy. The disadvantage of this technology is that it requires the use of reduced pressure which does not allow the work with continuous flow of gases.
On the other hand, U.S. Pat. No. 5,211,923 focuses a plasma discharge on the production of H2 and sulphur from acid gas that contains H2S residues and one or more COS, CS2 & SO2. The plasma type used in its process is R.F. operating in the microwave range and working within a pressure range of 0.5 to 2 atm. The temperature range reached with this type of plasma is from 150° C. to 450° C. Worth mentioning is the fact that this patent requires a pre-ionized Argon inlet system and a pre-heating system for the gas to be treated. Additionally, the system needs a cooling system that uses water that is not recycled but is released to the atmosphere.
Regarding CO2, the avant-garde technologies used for its degradation focus on those developed for cold plasma because of its excellent dissociation results. For example: the reduction of CO2 by Dielectric Barrier Discharge reported by Zheng et al. [Zheng G., Jiang J., Wu Y., Zhang R., Hou H., (2003), The Mutal conversion of CO2 and CO in Dielectric Barrier Discharge (DBD), Plasma Chemistry and Plasma Processing, 23, 59-68], and, especially, the reduction of CO2 using Gliding Arc Plasma in its non-equilibrium state reported by Idarto A. [Indarto A., (2007), Kinetics of CO2 Reduction by Gliding arc Plasma, Asian Journal of Water Environment and Pollution, 4, 191-194].
It is precisely the gliding arc discharge the one that offers more advantages for the decomposition of toxic and hazardous gases having a strong chemical structure. One example of gliding arc discharge applied in the treatment of H2S, is the one carried out by Czernichowski A. [Czernichowski A., (1998), Plasma pour valorization totale ou partiele des gaz contenant de L'H2S, Revue de L'Institut Fra{acute over (ç)}ais du Pétrole, 53, 163-179], who achieved an efficiency value of around 36% in the degradation of H2S using a power source based on an 8 kV, 50 Hz, 3-phase transformer with power consumption of approximately 2.4 kW.
In light of the above, it is therefore necessary to provide a method and a system to obtain sweet gas, synthetic gas, and sulphur from natural gas, through the use of highly selective membranes for CO2 & H2S in order to generate sweet gas, synthetic gas, and sulphur in a way that prevents pollution of the environment through the release of CO2 & SO2 into the atmosphere, and, at the same time, offer the possibility of transforming an acid gases stream into a synthetic gas steam and sulphur, which does not only allow for natural gas sweetening, but also offers the conversion of acid gases into value added products through the use of a hybrid plasma reactor.